1. Field of the Invention
This invention relates generally to the manufacture of semiconductor devices and, more particularly, to a process for forming a native oxide layer on the surface of a selected semiconductor material, at a low temperature and using non-ionizing radiation.
2. Description of the Prior Art
In the manufacture of semiconductor devices, it is often necessary to form an insulator layer on the surface of a semiconductor substrate to provide electrical insulation between adjacent layers or structures. In certain cases, it has been found that it is desirable to form a native oxide layer on the surface of the semiconductor substrate prior to the deposition of the insulator layer, in order to enhance the interface properties between the semiconductor and the insulator in the device formed therefrom. More specifically, both the surface state density at the semiconductor/insulator interface and the incorporation of fixed charge at this interface can be decreased by forming a native oxide layer on the surface of selected semiconductor substrates, such as indium phosphide, mercury cadmium telluride, or indium antimonide, prior to deposition of the insulator layer. The term "native oxide" is used herein to designate an oxide generated by the conversion of the top surface of the semiconductor substrate (approximately 10 to 100 angstroms) to the corresponding oxide. The term "fixed charge" is used herein to designate positive and negative uncompensated charges located in the native oxide due to defects, dangling bonds, or impurities generated during formation of the native oxide.
One method by which native oxide layers may be grown comprises a wet chemical anodization process, in which the semiconductor substrate to be coated with a native oxide is made the anode in an electrolytic cell, and a current is passed through a selected electrolyte, to thereby cause the native oxide layer to form on the semiconductor substrate. For example, in the formation of the native oxide of mercury cadmium telluride (HgCdTe), a suitable electrolyte would be a hydrogen peroxide and bromine solution or a hydrogen peroxide and acetone solution. Such an anodization process for the formation of a native oxide layer on a gallium arsenide substrate is described, for example, by L. Meiners, in the publication entitled "Surface potential of anodized p-GaAs MOS Capacitors," in Applied Physics Letters, Vol. 33, No. 8, 15 October 1978, pages 747-748. However, anodically grown native oxide layers, are, in many cases, unacceptable for device passivation purposes due to the incorporation of fixed and mobile charge in the native oxide. The term "mobile charge" is used herein to designate atmospherically generated contamination, such as sodium or potassium ions, which have a relatively high mobility through the insulator. In addition, in anodically grown native oxide layers, sodium ions or other impurity ions from the electrolytic bath may become incorporated in the native oxide layer formed. These fixed charges create high surface state densities (N.sub.ss) at the interface of the semiconductor with the native oxide/insulator composite in the subsequently formed device. (The term "native oxide/insulator composite" is used herein to designate the composite comprising the native oxide layer and the insulator layer formed thereon.) The high surface state densities at this latter interface will trap charges when a voltage is applied to the device, thereby preventing optimum device performance.
Another method by which a native oxide layer may be formed is a low temperature plasma process, in which reactive oxygen ions, for example, are produced by the action of an electric field, and the oxygen ions impinge on the semiconductor surface to cause the oxidation thereof, as described, for example, in U.S. Pat. No. 3,650,929 to K. Lertes and by N. Yokoyama et al. in the publication entitled "Low-temperature plasma oxidation of GaAs", in Applied Physics Letters, Vol. 32, No. 1, 1 January 1978, pages 58-60. Optionally, using a plasma anodization process, a bias voltage may be applied to the substrate during exposure to a plasma of oxygen ions to form a native oxide on the substrate, as described, for example, by L. A. Chesler and G. Y. Robinson, in the publication entitled "Plasma anodization of GaAs in a dc discharge", in the Journal of Vacuum Science and Technology, Vol. 15, No. 4, July/August 1978, pages 1525-1529. However, both plasma methods for native oxide growth discussed above impart charging and radiation damage to the semiconductor surface, and cause degraded semiconductor/insulator interface characteristics, such as high surface state density and incorporation of fixed charge. More specifically, in the previously described processes for the formation of a native oxide layer using a plasma of oxygen ions, in addition to the oxygen ions which are formed, numerous extraneous ionized and neutral particles, as well as high energy radiation with wavelengths as low as 500 angstroms and even extending into the soft x-ray region are produced and bombard the surface of the substrate on which the oxide is being formed. If the substrate comprises a sensitive device type, such as a charge-coupled device or a device formed of certain compound semiconductors (e.g. HgCdTe, InSb, or GaAs), the above-described charged particles and unwanted radiation frequently impart damage to these sensitive devices. In addition, the plasma may result in the incorporation of fixed charge in the insulator, which, in turn, will induce high surface state densities at the interface of the semiconductor with the native oxide/insulator composite, which will degrade device performance. Finally, thermal damage due to the selective absorption of radiation by the substrate and resultant heating thereof during plasma processing can lead to out-diffusion of one or more of the constituent elements of a compound semiconductor substrate, such as mercury in mercury cadmium telluride.
Thus, both the anodic process and the plasma process for forming a native oxide layer have the undesired effect of causing the incorporation of fixed or mobile charge in the native oxide so formed, which, in turn, contributes to a high surface state density at the interface of the semiconductor with the native oxide/insulator composite and degraded device performance. This prior art problem of the incorporation of fixed or mobile charge in the native oxide layer formed is due to exposure to charged particles or species (e.g. ions or electrons) or high energy radiation, and is caused by the manner in which the oxidizing species is formed and by the manner in which the native oxide layer is grown. It is the alleviation of this prior art problem of the incorporation of charge in a native oxide layer to which the present invention is directed.